As positive-led zoom lens systems, lens types are conventionally known in which a negative and a positive optical power are arranged on opposite sides of an aperture stop. For example, the zoom lens system proposed in Patent Document 1 listed below has a three-group—positive-negative-positive—zoom arrangement. There, the negative second group located on the front side of the aperture stop is divided into two negative groups, the distance between which is varied to achieve focusing. The zoom lens system proposed in Patent Document 2 listed below has a four-group—positive-negative-negative-positive—zoom arrangement. There, the part of the fourth group on the front side of the aperture stop serves as a focusing group. The zoom lens system proposed in Patent Document 3 listed below has a four-group—positive-negative-positive-positive—zoom arrangement. There, the positive third group located on the rear side of the aperture stop is divided into a positive and a negative group, and the negative group serves as a camera shake correction group.